Abstract:Two diterpene cyclases, one from the social amoeba Dictyostelium discoideum and the other from the bacterium Streptomyces clavuligerus, with products containing a Z-configured double bond between the original C2 and C3 of geranylgeranyl diphosphate, were extensively investigated for their mechanisms through isotopic labelling experiments. The participation of geranyllinalyl diphosphate, in analogy to the role of linalyl and nerolidyl diphosphate for mono- and sesquiterpene biosynthesis, as an intermediate towards diterpenes with a Z-configured C2=C3 double bond is discussed.